1. Field of the Invention
This invention relates to electrical shock and electrical arc fault protection circuits for electrical distribution systems and more particularly, to an improved protection system that can detect and eliminate many shock hazards and arcing faults within an electrical distribution system and which automatically resets itself upon the removal of the shock hazard and/or arcing fault.
2. Background of the Invention
Any electrical device ("the load") requires the flow of electrical current in order to operate. An analogy is the flow of water through an aquarium filter. A pump takes in water from the aquarium and increases the pressure (analogous to an increase in electrical voltage) to force the water through a tube (the tube is analogous to the electrical conductor or wire) to the filter (analogous to the electrical load). The flow of water current through the tube is analogous to the flow of electrical current in a wire. Most of the water pressure is "used up" in passing through the filter so that the water coming out of the filter has a relatively low pressure. A hose conveys the low pressure water from the filter outlet back to the aquarium. If the tube connecting the pump and the filter has a hole then some of the water will pass through this hole from the high pressure in the tube to a lower pressure outside the tube. This constitutes a water leak.
In somewhat the same way, an electrical device or load receives electrical energy from one terminal of an electrical outlet or source (the so-called high voltage or "hot" side), electrical current flows to the device through an electrical conductor or wire (the hot conductor), this current passes through the load and is then returned to another terminal of the electrical outlet through another wire called the neutral wire. The neutral wire will have a very low voltage (electrical pressure) because most of the voltage will be "used up" in the act of forcing electrical current through the load.
The two wires that connect source and load may have a coating of rubber or some other electrical insulating material or they may be bare, in which case air, which is a good insulator, functions to inhibit electrical current flow outside of the wire. Since the human body can conduct the flow of electrical current, if a person comes into contact with one electrified object such as the so-called hot wire in an electrical system, while also making contact with a second object having a substantially different voltage, then an electrical leakage current that is proportional to the voltage difference will flow through the person and may cause injury or death. If the second object that the person comes in contact with is electrically connected to the earth (ground) then this is a called a hot to ground fault. If the second object that the person comes in contact with is the neutral wire this is a hot to neutral fault. If the person makes contact between the neutral wire and ground this is called a neutral to ground fault. Since the voltage difference between neutral and ground is generally small (because the neutral line is connected to ground at the load center or breaker box where power is supplied), a neutral to ground fault is generally considered to be less hazardous than a hot to ground or a hot to neutral fault. Faults can also occur through the air. An undesirable electrical current leakage path through the air is generally manifested as a high energy electrical spark or "arc" and is referred to as an arc fault.
Electrical current is the flow of electrons. Electrons are neither created nor destroyed so any functioning electrical appliance will require both an entry path for electrons and an exit path for electrons in order for electrical current to flow. In an electrical appliance, electrons may exclusively enter on one path and exit on a second (direct current or DC operation). This is analogous to the aquarium leak example. For most household appliances that operate from a plug, electrons will sometimes enter path one and exit path two and sometimes enter path two and exit path one. This is known as alternating current or AC operation.
Although the two conductors coming out of an AC power source are often designated as "hot" and "neutral", in an AC system, the hot conductor will cyclically have a more positive voltage than the neutral for half the time and will cyclically have a more negative voltage than neutral for half the time, having a momentary value of zero each time the voltage passes from positive to negative and negative to positive.
A common source of electrical injuries in the home occurs when people place radios or similar electrical devices that are operated using household AC electrical current near their pool or bath tub while swimming or bathing. If the radio is knocked into the water, it can create electrical leakage current through the water to ground creating a ground fault. A ground fault can also occur when a person touches an electrically hot conductor while standing on or touching a grounded conductive surface. When sufficient current passes through a person, electrical burns or electrocution may result. Many electrical appliances such as heaters, hair dryers, electric razors and pumps are used near water and can present this type of hazard. Even a relatively low level of electrical current leakage can be hazardous to a human. Underwriters Laboratories, in their 943 standard for ground fault interrupt devices, requires that listed devices must open in response to any leakage current exceeding six milliamperes.
Another type of undesirable operating condition occurs when an electrical spark jumps between two conductors or from one conductor to ground. This spark represents an electrical discharge through the air and is objectionable because heat is produced as a byproduct of this unintentional "arcing" path. This occurrence is known as an arc fault and is a leading cause of, and contributor to, electrical fires. The electrical current levels drawn by an arcing fault are generally several orders of magnitude higher than the six milliampere standard for the Underwriters Laboratories 943 specification.
The U.S. Consumer Products Safety Commission estimates that there were approximately 41,000 fires involving electrical wiring systems in 1992. These fires resulted in 320 deaths, 1600 injuries and $511 million in property losses. The CPSC studies also show that the occurrence of wiring system fires is disproportionately high in homes more than 40 years old [Source: Technology for Detecting and Monitoring Conditions That Could Cause Electrical Wiring System Fires, Contract Number CPSC-C-94-1112, prepared by Underwriters Laboratories September 1995]. The disproportionately high incidence of wiring system fires in older homes may be attributed to poor wiring techniques, outdated electrical components and aged or damaged insulation on conductors. The present invention could be applied as a retrofit to older homes, affording protection against the arcing faults that are a major source of wiring system fires.
Arcing faults can occur in the same places that ground faults can occur--in fact, a ground fault would be called an arcing fault if it resulted in an electrical discharge across an air gap (a spark). As such, circuits that protect against ground faults can also prevent many classes of arcing faults. However, many of the circuits that purport to protect against arcing faults cannot detect low (albeit hazardous) level ground faults.
Protection circuits known as ground fault interrupters or GFI's, are presently required by code for the bathrooms of most new homes and commercial buildings. Similar circuits are also required for Underwriter's Laboratories approval in hair dryers sold in the United States. Of the GFI circuits presently available on the market, all use a current imbalance in a current sense transformer as the means of detecting a fault Such circuits are described, for example, in U.S. Pat. No. 3,683,302 (Butler et al) and U.S. Pat. No. 4,216,515 (Van Zeeland). One problem with approaches based upon a current sense transformer is that the magnetic core can acquire a residual magnetic state subsequent to one or more differential current conditions. This can result in a circuit which is too sensitive and is subject to nuisance tripping. In order to avoid nuisance trip problems, Underwriters Laboratories, in their 943 standard, requires that ground fault interrupt circuits not trip in response to a fault current of less than 4 milliamperes, thereby reducing the incidences of nuisance trippings. While existing ground fault interrupter circuits can detect and interrupt an arcing fault from the hot conductor to ground, they cannot detect and prevent an arcing fault that occurs between a hot conductor and a neutral conductor, a source of many electrical fires within homes and businesses. The present invention can detect such an occurrence within the household wiring. In addition, the present invention is an improvement over the current sense transformer based approaches in that it does not require the transformer and, since it is has a fast autoreset, nuisance tripping is not as great a concern and the device can be made responsive to a very low level of fault condition.
A spin-off of the conventional GFI circuit that has been applied to arc fault detection/avoidance in an appliance cordset is described in U.S. Pat. No. 4,931,894 (Legatti). This device works by using a conductive metal sheath to surround each current-carrying conductor individually. An arc protection winding is located on the core of the GFI current sense transformer and is connected in series with a resistance between the metal sheath and a neutral or return line. A fault involving any current carrying conductor will involve the shield before it involves any other conductive surface and the fault current will be sensed as a ground fault, thereby tripping the interrupting device in the plug. Problems with this approach, over and above the problems inherent in any current sense transformer based approach, is that this technology (U.S. Pat. No. 4,931,894) requires special wires that have a grounded conductive shield and this prevents its use in a system where it is desirable to use existing wiring.
The late 1980's and early 1990's saw a great deal of effort directed at arc fault detection/protection circuits. For example, U.S. Pat. No. 5,224,006 (Mackenzie and Engel) describes a system whereby the magnitude and rate of change of current is monitored. If the rate of change of current has a profile characteristic of a sputtering arc fault, a circuit breaker relay is tripped. U.S. Pat. No. 4,878,144 (Garin) uses a light sensitive arc detector to detect an arcing phenomenon and then trips a circuit breaker. U.S. Pat. Nos. 4,658,322 (Rivera) and 4,903,162 (Kopelman) use heat sensing elements to detect an overtemperature condition, such as that occuring due to an arcing fault, in electrical wiring and trigger a current interrupting circuit breaker in response thereto. U.S. Pat. No. 4,848,054 (Franklin) discloses a protective circuit that trips a circuit breaker upon the detection of an overload current condition which exceeds the maximum expected during normal transient conditions of operation, such overload current condition said to be characteristic of an arcing fault. The problems with all of the above cited arc fault protection technologies is that they cannot detect a low level (non-arcing) fault current which, although of relatively small value, can still result in a painful or even lethal electrical shock.
Additional arc fault detection circuits have been proposed that look for a specific signature characteristic of the current, voltage or electromagnetic field associated with arcing faults. These technologies concentrate on detecting a specific signature characteristic because many electric devices produce arcing during normal operations. An example is an electrical light switch which may draw a spark (an arc) when opened or an electric igniter for a gas furnace. Another example of a device that arcs in normal operation is a commutated motor which will spark continuously at the brushes when energized. An arc fault detector/interrupter would be useless if it detected and tripped in response to all arcs, both good (corresponding to normal operation) and bad (corresponding to a fault condition). Examples of technologies wherein the current flow is monitored, filtered and processed to detect an arcing fault include U.S. Pat. No. 4,639,817 (Cooper and South) wherein voltage signals are measured between the three phases in an AC three phase network and bandpass limited to the frequency band between 1 and 100 kilohertz, the band which is alleged to contain harmonic frequencies indicative of an arcing fault. U.S. Pat. Nos. 5,047,724 (Boksinger and Parente) and 5,280,404 (Ragsdale) reveal methods for detecting arc faults in an electrical circuit by comparing the spectral frequency makeup of voltages and/or currents within the circuit conductors to the spectrum characteristic of an arcing event. Examples of technologies wherein an electromagnetic field is monitored, filtered and processed to detect an arcing fault include U.S. Pat. Nos. 5,185,684 (Beihoff, Tennies, Richards and O'Neil), 5,185,685 (Tennies, Beihoff, Hastings, Clarey and O'Neil), 5,185,686 (Hansen, Beihoff, Tennies and Richards), 5,185,687 (Beihoff, Tennies, Richards and O'Neil) and 5,208,542 (Tennies, Beihoff, Hansen). The present invention is an improvement over the above referenced inventions in that the above inventions are directed at detecting an arcing condition but would be unresponsive to a low level fault current which, although not resulting in an arcing fault, could deliver a painful or lethal electrical shock if the fault path was a human body. Furthermore, most of the filtering algorithms proposed by these arc fault directed inventions require a signal analysis over multiple cycles and cannot detect and respond to a fault in less than one cycle of the fundamental AC source frequency.
3. Objects and Advantages
The present invention has the following objects and advantages:
a) requires only two electrical conductors connecting the parts of an electrical distribution system (no ground wire required); PA1 b) does not use a current sense transformer for fault detection; PA1 c) can detect and interrupt arcing faults from the hot conductor to ground; PA1 d) can detect and interrupt shock hazards from the hot conductor to ground; PA1 e) can detect and interrupt an arcing fault from the hot conductor to the neutral conductor within the wiring connecting the load center and the outlets/lights in an electrical distribution system; PA1 f) interrupts current flow within one half cycle from the occurrence of a fault; PA1 g) electrical power is automatically restored to the device upon removal of the fault (auto-reset); PA1 h) can be easily retrofit into existing houses or other installations without requiring a restringing of existing wiring within that house or other installation; PA1 i) can be used to detect and prevent an open ground condition or miswired neutral at wall outlets within a home; PA1 j) can be made to provide short circuit current limiting and protection.
Further objects and advantages of the present invention will become apparent from a description of the drawings and ensuing description. None of the prior art patents described in section 2, either alone or in any appropriate combination, anticipates or renders the disclosed invention unpatentable.